Composite differential direct-coupled instrumentation amplifier

ABSTRACT

A differential amplifier which utilizes a pair of wideband operational amplifiers as output stages for two differential output signals, a pair of high performance DC operational amplifiers to buffer input signals to the pair of wideband operational amplifiers used as output stages and reduce offset voltage error by the amount of the DC gain of the pair of high performance DC operational amplifiers, apparatus for biasing the pair of high performance DC operational amplifiers to provide exceptional DC characteristics and eliminate common mode response from those amplifiers, and apparatus for feeding forward high frequency AC input to the wideband operational amplifiers operating as the output stages without loading the common mode input impedance of the differential amplifier.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

This invention relates to differential amplifiers and, moreparticularly, to circuitry utilizing DC operational amplifiers to form acomposite differential direct-coupled instrumentation amplifier havingboth high DC accuracy and wide bandwidth.

2. History Of The Prior Art

A typical prior art instrumentation amplifier is implemented with threeoperational amplifiers. Two of the operational amplifiers each receiveone of the two differential input signals the difference between whichis to be measured; these two operational amplifiers each provide highclosed loop gains. The two output signals generated by these operationalamplifiers are furnished as the input signals to the third operationalamplifier which utilizes balanced feedback and input resistors so thatit operates at a closed loop gain of one. Because it operates at a gainof one, it may provide a relatively high bandwidth of approximately onemegahertz and little DC error.

Several types of operational amplifiers may be used in such a circuitwith varying performance characteristics. However, devices which providegood DC performance do not provide closed loop bandwidths above a fewkilohertz. Because of their limited bandwidth, they also exhibit poor ACcharacteristics and have poor AC common mode rejection at frequencies aslow as 60 hertz. Conversely, operational amplifiers which can provideclosed-loop bandwidths approaching one megahertz have DC errors severalorders of magnitude higher than the precision DC devices.

A high performance instrumentation amplifier, intended formulti-frequency use, should provide both good DC performance as well asfrequency response and well controlled phase shift characteristics fromDC to beyond 500 kilohertz. By comparing these statistics with those ofa typical prior art instrumentation amplifier, it may be seen that thetypical instrumentation amplifier does not provide these results.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide improvedinstrumentation amplifiers.

It is another more specific object of the present invention to providean instrumentation amplifier with a frequency response from DC to over500 kilohertz exhibiting minimal DC offset, little bias current, highcommon mode rejection, and high closed loop gain.

These and other objects of the present invention are realized in adifferential amplifier which utilizes a pair of wideband operationalamplifiers as the output stages for the two differential output signals,a pair of high performance DC operational amplifiers to buffer the inputsignals to the output operational amplifiers and reduce the offsetvoltage error by the amount of their DC gain, means for biasing the pairof high performance DC operational amplifiers to provide exceptional DCcharacteristics and eliminate common mode response from thoseamplifiers, and means for feeding forward high frequency AC input to thewideband operational amplifiers operating as the output stages.

These and other objects and features of the invention will be betterunderstood by reference to the detailed description which follows takentogether with the drawings in which like elements are referred to bylike designations throughout the several views.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a prior art instrumentationamplifier.

FIG. 2 is a circuit diagram of an instrumentation amplifier inaccordance with the present invention.

FIG. 3 is another circuit diagram of an instrumentation amplifier inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is illustrated a circuit diagram of aninstrumentation amplifier 10 designed in accordance with the prior art.The instrumentation amplifier 10 includes first and second matchedoperational amplifiers 12 and 13. The operational amplifier 12 receivesthe high end of the differential input at its non-inverting inputterminal, and the operational amplifier 13 receives the low end of thedifferential input at its non-inverting input terminal. The operationalamplifier 12 receives feedback at its inverting input terminal from itsoutput terminal via a resistor 15 while the operational amplifier 13receives feedback at its inverting input terminal from its outputterminal via a resistor 16. A resistor 17 joins the inverting terminalsof the two operational amplifiers 12 and 13.

The output of the operational amplifier 12 is furnished to thenon-inverting input terminal of a third operational amplifier 20 by aresistor 21. The output of the operational amplifier 13 is furnished tothe inverting input terminal of the third operational amplifier 20 by aresistor 22. A resistor 23 connects the non-inverting input terminal ofthe operational amplifier 20 to ground, and a resistor 24 connects theinverting input terminal of the operational amplifier 20 to its outputto provide feedback. The output signal is taken between the outputterminal of the operational amplifier 20 and ground.

Typically, the resistors 21, 22, 23, and 24 are all carefully selectedto have equal values so that the stage including the operationalamplifier 20 has a gain of one. This allows the last stage to havecharacteristics which provide good operation at higher frequencies andto accept a relatively wide bandwidth. The balanced resistors 21-24provide significant common mode rejection properties. Even though theamplifier 20 is not designed to have good DC characteristics, the DCerror is kept to a minimum because the amplifier 20 provides a gain ofonly one.

All of the gain of the instrumentation amplifier 10 is provided by thefirst stage which includes the two high precision operational amplifiers12 and 13. The gain for the circuit 10 is equal to 1+((R1+R3)/R2), whereRA=RB=RC=RD. The first stage typically provides gain for the circuit 10of from one to one thousand, and amplifies input signals from a fewmillivolts to ten volts. As pointed out above, the input amplifiers 12and 13 are typically matched operational amplifiers having very good DCcharacteristics. Thus, the DC input signals handled by the circuit 10are provided to the operational amplifier 20 with little error.

A number of operational amplifiers are available as off-the-shelf itemswhich may be used in the circuit 10. Many of these operationalamplifiers provide good DC characteristics such as offset voltages ofunder ten microvolts with temperature coefficients of less than 0.25uV/degree centigrade, and input bias currents of less than 0.25nanoamperes over wide temperature ranges with very low noise. However,operational amplifiers which provide good DC performance do not provideclosed loop bandwidth much above a few kilohertz.

The output of the amplifiers 12 and 13 are provided to the amplifier 20.Although an operational amplifier such as amplifier 20 can provideclosed-loop bandwidths approaching one megahertz, such amplifiers haveDC errors several orders of magnitude higher than the precision DCdevices. However, the errors produced at the DC end of the frequencyspectrum are not magnified by the amplifier 20 because of its low gain.The amplifier 20 provides fairly decent common mode rejection as wellbecause of its balanced resistor network. However, the input amplifiers12 and 13 are not able to handle higher frequencies accurately, so theinput signals at higher frequencies provided to the amplifier 20 containhigh frequency distortion. Because of this, the circuit 10 cannotproduce the desired specifications which have been outlined above. Infact, such a circuit cannot be expected to provide a closed loopbandwidth much above a few kilohertz.

To provide both wide bandwidth and excellent DC characteristics, acircuit 30 of FIG. 2 has been devised in accordance with the presentinvention. The circuit 30 includes a pair of matched output operationalamplifiers U1 and U2. Each of the output amplifiers is chosen and biasedto provide wide band operation. To this end, biasing is provided bysources Vcc and -Vcc which are typically 15 volts.

A pair of matched high precision operational amplifiers U3 and U4receive the differential input signals at their non-inverting inputterminals, amplify those signals, and provide input signals to thenon-inverting input terminals of the amplifiers U1 and U2, respectively.The amplifiers U3 and U4 are chosen to provide excellent DC performanceand high gain. The output of the amplifiers U1 and U2 are fed back tothe inverting input terminal of the amplifiers U3 and U4. Since the highgain amplifiers U3 and U4 tend to produce an output to bring theirinverting input terminals to the same voltage as the non-inverting inputterminals, the offset voltage error of the amplifiers U1 and U2 isreduced by the amount of the DC gain of the amplifiers U3 and U4.

In order to assure the precision DC characteristics of the amplifiers U3and U4, floating power supplies of plus and minus two volts are providedin the preferred embodiment of the circuit 30. The amplifiers U3 and U4are biased from very small floating voltage sources such as the two (twovolt) batteries illustrated in the figure. Because the amplifiers U3 andU4 are biased from floating potential sources rather than Vcc and minusVcc, these amplifiers operate independently of the input signal leveland are not subjected to common mode input voltages. The common modeinput voltages are passed through to the amplifiers U1 and U2.

Feedback from the operational amplifier U1 is buffered by an additionaloperational amplifier U5. Feedback from the operational amplifier U2 isbuffered by an additional operational amplifier U6. Each of theamplifiers U5 and U6 are connected as voltage followers with gain = +1.Since the non-inverting input terminals of the amplifiers U5 and U6 areconnected to receive the feedback from the output terminal of U1 and U2as are the inverting input terminals of the amplifiers U3 and U4,respectively, the voltage at the non-inverting input to the amplifiersU5 and U6 moves with the voltage at the inverting terminals of theamplifiers U3 and U4 to seek the same level as the non-invertingterminals of the amplifiers U3 and U4. Since the non-inverting input tothe amplifiers U5 and U6 moves with the non-inverting input of theamplifiers U3 and U4 and because the amplifiers U5 and U6 have a gain ofone, the output terminals of the amplifiers U5 and U6 move with thevoltage at the non-inverting input terminals of the amplifiers U3 andU4. The output terminals of these amplifiers U5 and U6 are connectedbetween the two batteries which are the floating power supplies for theamplifiers U3 and U4. Consequently, the voltage level between the fixedtwo volt power supplies moves (as do the output terminals of theamplifiers U5 and U6) with the input to the amplifiers U3 and U4; andthe amplifiers U3 and U4 do not see the common mode voltage movementswith respect to the sources Vcc and -Vcc. Thus, the precision DCamplifiers U3 and U4 are not subject to common mode voltages.

Since the amplifiers U3 and U4 are not subject to the common mode inputvoltages which are passed on to the amplifiers U1 and U2, the maximumcommon mode swing is limited by the voltage swing capabilities of theamplifiers U1, U2, U5, and U6, all of which are biased by the values ofVcc and minus Vcc (typically 15 v. and -15 v.).

Input signals to the circuit 30 are provided at the non-inverting inputterminals of the amplifiers U3 and U4. Because the amplifiers U3 and U4are chosen to have excellent DC characteristics, input signals at the DCend of the frequency spectrum are handled accurately and transferred intheir amplified condition to the wide band amplifiers U1 and U2 so thatthe composite amplifier 30 may be made to respond very well to lowfrequency input signals.

However, higher frequencies are subject to distortion by the amplifiersU3 and U4 which are tuned to handle DC levels with precision; andamplification of these higher frequencies is attenuated by approximatelysix decibels per octave beginning at a frequency which varies with thecharacteristics of the amplifiers U3 and U4 but may be about 0.1 hertz.In order to provide the desired wide band characteristics, a pair ofcapacitors C2 and C2# provide the high frequency input signals furnishedat the differential input terminals to the circuit 30 to the amplifiersU1 and U2. The capacitors C2 and C2# have values selected and arearranged to transfer the higher frequency input signals around theamplifiers U3 and U4 to the non-inverting input terminals of theamplifiers U1 and U2. In this manner, the higher frequency values aretransferred to the stages U1 and U2 which are capable of transferringthose signals with low distortion. In order to assure that the highfrequency input signals which bypass the amplifiers U3 and U4 are notloaded, resistors R5 and R5# are provided. Since the output of theamplifier U5 is not only arranged to bias the center of the floating(two volt) power supplies but is also connected to the inverting inputto the amplifier U1, the voltage at the inverting input to the amplifierU1 moves with input to the non-inverting terminal to the amplifier U3.Since voltage at the inverting terminal of amplifier U1 tends to followthe non-inverting terminal, this places the same voltage on both sidesof the capacitor C2 to assure that the amplifier U1 is not loaded by thecapacitor C2. The amplifier U6 operates similarly with respect to thecapacitor C2#. Consequently, the common mode input impedance of theoverall differential amplifier is not loaded by the bypass arrangement.

Although the output of the circuit 30 illustrated is a differentialoutput, a final stage such as that illustrated in FIG. 1 including theamplifier 20 could be added at the output terminals and utilized toprovide a single ended output referred to ground.

FIG. 3 illustrates another circuit 40 constructed in accordance with thepresent invention. The circuit 40 is very similar to the circuit 30illustrated in FIG. 2 but is the presently preferred embodiment of theinvention; for this reason, values of circuit elements necessary for anoperating circuit are provided in the figure. The circuit 40 includes apair of output operational amplifiers U1 and U2. Each of the outputamplifiers is chosen to provide wide band operation. To this end,National Semiconductor operational amplifiers LM6361 (manufactured byNational Semiconductor Co., 2900 Semiconductor Drive, Santa Clara,Calif. 95052) or similar devices may be used.

A pair of high precision operational amplifiers U3 and U4 which arematched for offset voltage provide input signals to the non-invertinginput terminals of the amplifiers U1 and U2, respectively. Theamplifiers U3 and U4 are chosen to provide excellent DC performance andhigh gain. The amplifiers U3 and U4 essentially buffer the input signalsprovided to amplifiers U1 and U2 and reduce the DC offset voltage errorof the amplifiers U1 and U2 by the amount of their DC gain. In order toprovide these characteristics, a pair of Linear Technology operationalamplifiers LT1012 (manufactured by Linear Technology Co., 1630 McCarthyBlvd., Milpitas, CA. 95035) or similar devices may be used.

In order to assure the precision DC characteristics of the amplifiers U3and U4, floating power supplies providing plus and minus two volts areprovided in the preferred embodiment of the circuit 40. These powersupplies are furnished by a pair of field effect transistors Q1 and Q2which are connected, respectively, to Vcc and -Vcc values of 15 volts.The field effect transistors Q1 and Q2 have their gate terminals joiningtheir source terminals so that they are biased into the saturationcondition (IDss) and each acts as a constant current source. A pair ofzener diodes CR1 and CR2 join the source terminal of transistor Q1 tothe drain terminal of the transistor Q2 to provide a constant floatingplus and minus two volts to supply operating power to the amplifiers U3and U4. Because the amplifiers U3 and U4 are biased from floatingpotential sources rather than Vcc and minus Vcc, these amplifiersoperate independent of input signal level and any common mode inputvoltages. The common mode input voltages are passed through to theamplifiers U1 and U2.

Feedback from the operational amplifier U1 is buffered by an additionaloperational amplifier U5. Feedback from the operational amplifier U2 isbuffered by an additional operational amplifier U6. Each of theamplifiers U5 and U6 are connected as gain +1 voltage followers. Theoutput terminals of these amplifiers U5 and U6 are connected between thezener diodes CR1 and CR2 and thus drive the value of the source voltagesapplied to the amplifiers U3 and U4 to maintain a floating power supplylevel for those amplifiers U3 and U4. To this end, NationalSemiconductor operational amplifiers LM310 (manufactured by NationalSemiconductor Co., 2900 Semiconductor Drive, Santa Clara, Calif. 95052)or similar devices may be used for the amplifiers U5 and U6.

Since the amplifiers U3 and U4 are not subject to the common mode inputvoltages which are passed on to the amplifiers U1 and U2, the maximumcommon mode swing is limited by the voltage swing capabilities of theamplifiers U1, U2, U5, and U6, all of which are biased by the values ofVcc and minus Vcc (typically 15 v. and -15 v.).

Because the amplifiers U3 and U4 are chosen to have excellent DCcharacteristics and the amplifiers U1 and U2 are wide band devices, thecomposite amplifier may be made to respond very well to low frequencyinput signals. Moreover, capacitors C2 and C2# are arranged to transferthe higher frequency input signals around the amplifiers U3 and U4 tothe non-inverting input terminals of the amplifiers U1 and U2. In thismanner, the higher frequency values are transferred to the stages U1 andU2 which are capable of transferring those signal with low distortion.In order to assure that the high frequency input signals which bypassthe amplifiers U3 and U4 are not loaded, a parallel arrangementincluding a capacitor C4 and a resistor R5 is provided. The capacitor C4and the resistor R5 are chosen to provide a tuned RC network with theinternal values of the two amplifiers U1 and U3 so that a stablefeedback loop is obtained for the circuit 40. The values used for theresistor R5 and the capacitor C4 will, of course, depend on theparticular amplifiers U1 and U3 used in the circuit 40.

The values of circuit elements not otherwise discussed in thisspecification are given in FIG. 3. A circuit 40 such as that illustratedin FIG. 3 provides the following characteristics:

Closed loop gain x100

Full scale output +/-10 volts

Common mode range +/-10 volts

Frequency response DC to 750 kilohertz

Slew rate 40 volts/uSec.

DC offset +/-10 uV RTI

DC offset vs. temp. +/-0.25 uV/degree C RTI

Bias current +/-0.25 nA.

Bias current vs. temp. +/-0.05 nA/degree C.

Although the present invention has been described in terms of apreferred embodiment, it will be appreciated that various modificationsand alterations might be made by those skilled in the art withoutdeparting from the spirit and scope of the invention. The inventionshould therefore be measured in terms of the claims which follow.

What is claimed is:
 1. A differential instrumentation amplifiercomprising a pair of wideband operational amplifiers as output stagesfor two differential output signals, a pair of high performance DCoperational amplifiers to buffer input signals to the output operationalamplifiers and reduce offset voltage error by the amount of their DCgain, means for biasing the pair of high performance DC operationalamplifiers to provide precision DC characteristics and eliminate commonmode responses from the high performance DC operational amplifiers, andmeans for feeding forward high frequency AC input to the widebandoperational amplifiers operating as output stages while eliminatingcommon mode voltages in the high frequencies, in which the means forbiasing the pair of high performance DC operational amplifiers toprovide precision DC characteristics and eliminate common mode responsefrom the high performance DC operational amplifiers comprises first andsecond floating voltage sources for applying voltage levels to each ofthe high performance DC operational amplifiers, means joining oneterminal of each of the first and second floating voltage sourcesassociated with each high performance DC operational amplifier to abiasing terminal of the high performance DC operational amplifier, andmeans for varying the voltage level at a second terminal of the firstand second floating voltage sources to match a level at a non-invertinginput to the associated amplifier.
 2. A differential instrumentationamplifier as claimed in claim 1 in which the first and second floatingvoltage sources provide voltage at a lower level than biasing levelsprovided for the pair of wideband operational amplifiers.
 3. Adifferential instrumentation amplifier as claimed in claim 1 in whichthe first and second floating voltage sources each comprise a zenerdiode, and a source of constant current for the zener diode.
 4. Adifferential instrumentation amplifier as claimed in claim 1 in whichthe means for varying the voltage level of the second terminal of eachof the first and second floating voltage sources to match a level at anon-inverting input to the associated amplifier comprises a unity gainoperational amplifier having an output terminal connected to the secondterminal of each of the first and second floating voltage sources, andmeans for providing a voltage level at a non-inverting input to theassociated high performance DC operational amplifier to a non-invertinginput terminal of the operational amplifier having unity gain.
 5. Adifferential instrumentation amplifier comprising a pair of widebandoperational amplifiers as output stages for two differential outputsignals, a pair of high performance DC operational amplifiers to bufferinput signals to the output operational amplifiers and reduce offsetvoltage error by the amount of their DC gain, means for biasing the pairof high performance DC operational amplifiers to provide precision DCcharacteristics and eliminate common mode responses from the highperformance DC operational amplifiers, and means for feeding forwardhigh frequency AC input to the wideband operational amplifiers operatingas output stages while eliminating common mode voltages in the highfrequencies, in which the means for feeding forward high frequency ACinput to the wideband operational amplifiers operating as output stagescomprises a capacitor joining a non-inverting input terminal of each ofthe high performance DC operational amplifiers with a non-invertinginput terminal of each of the output operational amplifiers.
 6. Adifferential instrumentation amplifier as claimed in claim 5 in whichthe means for biasing the pair of high performance DC operationalamplifiers to provide precision DC characteristics and eliminate commonmode response from the high performance DC operational amplifierscomprises first and second floating voltage sources for applying voltagelevels to each of the high performance DC operational amplifiers, meansjoining one terminal of each of the first and second floating voltagesources associated with each high performance DC operational amplifierto a biasing terminal of the high performance DC operational amplifier,and means for varying the voltage level at a second terminal of thefirst and second floating voltage sources to match a level at anon-inverting input to the associated amplifier.
 7. A differentialinstrumentation amplifier as claimed in claim 6 in which the first andsecond floating voltage sources comprise a pair of batteries.
 8. Adifferential instrumentation amplifier as claimed in claim 6 in whichthe first and second floating voltage sources each comprise a zenerdiode, and a source of constant current for the zener diode.
 9. Adifferential instrumentation amplifier as claimed in claim 6 in whichthe means for varying the voltage level of the second terminal of eachof the first and second floating voltage sources to match a level at anon-inverting input to the associated amplifier comprises a unity gainoperational amplifier having an output terminal connected to the secondterminal of each of the first and second floating voltage sources, andmeans for providing a voltage level at a non-inverting input to theassociated high performance DC operational amplifier to a non-invertinginput terminal of the operational amplifier having unity gain.
 10. Adifferential instrumentation amplifier comprising a pair of widebandoperational amplifiers as output stages for two differential outputsignals, a pair of high performance DC operational amplifiers to bufferinput signals to the wideband output operational amplifiers and reduceoffset voltage error by the amount of the DC gain of the widebandamplifiers, and means for biasing the pair of high performance DCoperational amplifiers to provide precision DC characteristics andeliminate common mode responses from the high performance DC operationalamplifiers, the means for biasing including first and second floatingvoltage sources for biasing each of the high performance DC operationalamplifiers, means joining one terminal of each of the first and secondfloating voltage sources associated with each high performance DCoperational amplifier to a biasing terminal of the high performance DCoperational amplifier, and means for varying the voltage level at asecond terminal of the first and second floating voltage sources tofollow an input signal level at an input to the associated highperformance DC operational amplifier.
 11. A differential instrumentationamplifier as claimed in claim 10 in which the means for varying thevoltage level at a second terminal of the first and second floatingvoltage sources to follow an input signal level at an input to theassociated high performance DC operational amplifier comprises:feedbackmeans coupling the output of the each wideband operational amplifier tothe second terminal of the first and second voltage sources of the highperformance DC operational amplifier joined to a non-inverting input ofthat wideband operational amplifier, and in which the feedback means isconnected to an inverting input of the high performance DC operationalamplifier which is joined to the non-inverting input of that widebandoperational amplifier.